In the technical field of optical communications, there are increasing demands for diversification of services and higher data transmission rates. In these years, wavelength multiplexing optical transmission systems of a transmission rate higher than 40 Gbps per wave are coming closer to realization. Unlike for optical signal transmission of around 10 Gbps, which is popular at present, no standard scheme has been determined for high-speed optical signal transmission as fast as 40 Gbps. In optical signal transmission of a transmission rate higher than or equal to 10 Gbps, not only is signal quality degraded by optical noise in optical amplifiers, but also optical signal characteristics are restricted by chromatic dispersion, non-linear effects, polarization mode dispersion, spectral narrowing by optical filters, etc., in transmission paths. Therefore, research and development have been promoted of modulation techniques with narrow spectral width that improve dispersion tolerance and non-linearity tolerance. In particular, in 40-Gbps wavelength multiplexing optical transmission systems, not only those using NRZ (Non-Return to Zero) but also those using modulation techniques such as CSRZ (Carrier-Suppressed Return to Zero), DPSK (Differential Phase Shift Keying), and DQPSK (Differential Quadrature Phase Shift Keying) have been proposed. Thus, systems using various modulation techniques are expected to be available in the future. For example, a 10-Gbps WDM (Wavelength Division Multiplexing) system, which is common at present, is described in K. Nakamura et al., “1.28 Tbit/s Transmission over 1680 km Standard SMF with 120 km Optical Repeater Spacing Employing Distributed Raman Amplification,” OECC2000, PD1-7.